US6845212B2 - Optical element having programmed optical structures - Google Patents

Abstract

An optical element, such as a lightguide or an optical film, is formed with a predetermined, programmed pattern of optical structures. The optical structures may be arranged to selectively correct for non-uniformities in the output a lightguide, or may be arranged to otherwise enhance the performance of a display.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to optical elements and more particularly to lightguides, optical films and other optical elements suitable for use in display devices and having programmed optical structures.

2. Description of the Related Technology

Backlit display devices, such as liquid crystal display (LCD) devices, commonly use a wedge-shaped lightguide. The wedge-shaped lightguide couples light from a substantially linear source, such as a cold cathode fluorescent lamp (CCFL), to a substantially planar output. The planar output is then used to illuminate the LCD.

One measure of the performance of the backlit display is its uniformity. A user can easily perceive relatively small differences in brightness of a display from one area of the display to the next. Even relatively small non-uniformities can be very annoying to a user of the display.

Surface diffusers or bulk diffusers, which scatter the light exiting the lightguide, are sometimes used to mask or soften non-uniformities. However, this diffusion also results in light being directed away from a preferred viewing axis. A net result can be a reduction in overall brightness of the display along the preferred viewing axis, which is another performance measure of a display device.

Unlike non-uniformities, from a subjective standpoint relatively small increases or decreases in overall brightness are not easily perceived by the user of the display device. However, the display device designer is discouraged by even the smallest decreases in overall brightness including decreases so small they might only be perceived by objective measurement. This is because display brightness and power requirements of the display are closely related. If overall brightness can be increased without increasing the required power, the designer can actually allocate less power to the display device, yet still achieve an acceptable level of brightness. For battery powered portable devices, this translates to longer running times.

SUMMARY OF THE INVENTION

In accordance with the invention, an optical element, such as a lightguide or an optical film, is formed with a predetermined, programmed pattern of optical structures. The optical structures may be arranged to selectively correct for non-uniformities in the output of a lightguide, or may be arranged to otherwise effect the performance of the display in a predetermined, and designed manner.

In a first aspect of the invention, an optically transmissive film having a first surface and a second surface and a first edge and a second edge is formed with a plurality of optical structures formed in the first side. The plurality of optical structures are arranged on the first side in a predetermined pattern, and each optical structure has at least one characteristic selected from the group consisting of an amplitude, a period and an aspect ratio. Each characteristic has a first value for a first predetermined location on the film between the first edge and the second edge and the characteristic has a second value, different from the first value, for a second predetermined location on the film, different than the first predetermined location on the film, between the first edge and the second edge.

In another aspect of the invention, the structure in accordance with the invention is part of a thick optical element, such as for example, a lightguide wedge. The structure is achieved on the thick element through injection molding, compression molding, or by bonding a film with the structure to the additional optical element.

BRIEF DESCRIPTION OF THE DRAWINGS

The many advantages and features of the present invention will become apparent to one of ordinary skill in the art from the following detailed description of several preferred embodiments of the invention with reference to the attached drawings wherein like reference numerals refer to like elements throughout and in which:

FIG. 1 is a perspective view of an illumination device adapted in accordance with an embodiment of the invention;

FIG. 2 is a perspective view of an optical film incorporating a programmed pattern of optical structures in accordance with one embodiment of the invention;

FIG. 3 is a perspective view of an optical film incorporating a programmed pattern of optical structures in accordance with another embodiment of the invention;

FIG. 4 is a perspective view of a lightguide wedge incorporating an in-phase programmed pattern of optical structures in accordance with another embodiment of the invention;

FIG. 5 is a cross-section view taken alongline5—5 of inFIG. 4;

FIG. 6 is a perspective view of a lightguide wedge incorporating an out-of-phase programmed pattern of optical structures in accordance with another embodiment of the invention;

FIG. 7 is perspective view of a linear lens structure incorporating a programmed pattern of optical structures in accordance with another embodiment of the invention;

FIG. 8 is a perspective view of an optical film incorporating a programmed pattern of optical structures in accordances with an alternate preferred embodiment of the invention;

FIG. 9 is a perspective view of an optical film incorporating a programmed pattern of optical structures in accordances with an alternate preferred embodiment of the invention;

FIG. 10 is a perspective view of an optical film incorporating a programmed pattern of optical structures in accordances with an alternate preferred embodiment of the invention;

FIG. 11 is a side view of a lightguide incorporating first programmed pattern of optical structures in a top surface and a second programmed pattern of optical structures in a bottom surface in accordance with a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention is described in terms of several preferred embodiments, and particularly, in terms of an optical film or a lightguide suitable for use in a backlighting system typically used in flat panel display devices, such as a laptop computer display or a desktop flat panel display. The invention, however, is not so limited in application and one of ordinary skill in the art will appreciate that it has application to virtually any optical system, for example, to projection screen devices and flat panel televisions. Therefore, the embodiments described herein should not be taken as limiting of the broad scope of the invention.

Referring toFIG. 1, anillumination system10 includes alight source12; alight source reflector14; alightguide16 with anoutput surface18, aback surface20, aninput surface21 and anend surface22; areflector24 adjacent theback surface20; a firstlight redirecting element26; a secondlight redirecting element28; and areflective polarizer30. Thelightguide16 may be a wedge, a modification thereof or a slab. As is well known, the purpose of the lightguide is to provide for the distribution of light from thelight source12 over an area much larger than thelight source12, and more particulary, substantially over an entire area formed byoutput surface18. Thelightguide16 further preferably accomplishes these tasks in a compact, thin package.

Thelight source12 may be a CCFL that inputs light to theedge surface21 of thelightguide16, and thelamp reflector14 may be a reflective film that wraps around thelight source12 forming a lamp cavity. Thereflector24 backs thelightguide16 and may be an efficient back reflector, e.g., a lambertian film or a specular film or a combination.

In the embodiment shown, the edge-coupled light propagates from theinput surface21 toward theend surface22, confined by total internal reflection (TIR). The light is extracted from thelightguide16 by frustration of the TIR. A ray confined within thelightguide16 increases its angle of incidence relative to the plane of the top and bottom walls, due to the wedge angle, with each TIR bounce. Thus, the light eventually refracts out of theoutput surface18 and at a glancing angle thereto, because it is no longer contained by TIR. Some of the light rays are extracted out of theback surface20. These light rays are reflected back into and through thelightguide16 by theback reflector24. Firstlight redirecting element26 is arranged as a turning film to redirect these light rays exiting theoutput surface18 along a direction substantially parallel to a preferred viewing direction.

As shown inFIG. 2, the firstlight redirecting element26 may be a light transmissive optical film with anoutput surface32 and aninput surface34 formed with prisms (not shown), which refract and reflect the light exiting thelightguide16 along the preferred viewing direction. The prisms may have a substantially uniform configuration, or may have a non-uniform configuration as described in commonly assigned U.S. patent application Ser. No. 09/415,873 “OPTICAL FILM WITH VARIABLE ANGLE PRISMS” filed of even date herewith, the disclosure of which is hereby expressly incorporated herein by reference.

Referring back toFIG. 1, the secondlight redirecting element28 may not be required in every configuration of theillumination system10. When included in thesystem10, the second light redirecting element may be a diffuser, a lenticular spreader or a prism film, for example a brightness enhancing film such as the 3M Brightness Enhancement Film product (sold as BEFIII) available from Minnesota Mining and Manufacturing Company, St. Paul, Minn. Thereflective polarizer30 may be an inorganic, polymeric or cholesteric liquid crystal polarizer film. A suitable film is the 3M Diffuse Reflective Polarizer Film product (sold as DRPF) or the Specular Reflective Polarizer film product (sold as DBEF), both of which are available from Minnesota Mining and Manufacturing Company. Furthermore, at least the secondlight redirecting element28 and thereflective polarizer30, and potentially the firstlight redirecting element26, may be combined into a single optical element. The commonly assigned U.S. patent application Ser. No. 09/415,100 entitled “DISPLAY ILLUMINATION DEVICE AND METHOD OF ENHANCING BRIGHTNESS IN A DISPLAY ILLUMINATION DEVICE” filed of even date herewith, the disclosure of which is hereby expressly incorporated herein by reference, describes several such combined optical structures.

With lightguides used for backlighting, such aslightguide16, it is common for there to be non-uniformities in the light output from the lightguide. These non-uniformities can frequently be concentrated near theinput surface21. To mask these defects in applications of the lightguide, a diffuser that covers the output surface of the lightguide is typically used. However, a diffuser tends to reduce the overall brightness of the display and may not adequately mask all of the defects.

Referring now toFIG. 2, shown graphically is a film containing an in-phase varying amplitude pattern. The pattern described may be formed on a top or bottom surface of a wedge, on a plano film, or as described below, on a turning film. In that regard, in addition to the prisms formed on theinput surface34 of the firstlight redirecting element26, theoutput surface32 may be formed with optical structures. More particularly, the firstlight redirecting element26 has afirst edge36 and asecond edge38. Extending from thefirst edge36 toward thesecond edge38 are a plurality ofoptical structures40 arranged in apattern42. Eachoptical structure40 may have a number of characteristics, such as amplitude, period and aspect ratio of thepeaks44 andvalleys46. Thepattern42 may also have characteristics, such as for example, a pitch, p, betweenoptical structures40. Thestructures40 inFIG. 2 are shown having amplitude variation. In application of the firstlight redirecting structure26, the grooves may be arranged such that variation in amplitude is perpendicular to thelightsource12.

With continued reference toFIG. 2, it is observed that within thepattern42, theoptical structures40 are formed with larger amplitude A1 at thefirst edge36 and decrease in amplitude A2 toward thesecond edge38. The larger amplitude A1 produces more optical power along the groove axis,1, because of the higher surface slopes. The optical power of this pattern then decreases as a function of the distance from thefirst edge36. This arrangement of theoptical structures40 and thepattern42 is purposeful. As noted, non-uniformities in the output oflightguide16 may be concentrated near theinput surface21 while there may be less non-uniformity farther from theinput surface21. Thus, theoptical structures40 and thepattern42 are arranged to provide more diffusion nearfirst edge36. In application,first edge36 will be disposed substantially adjacent theinput surface21 of thelightguide16.Pattern42 may have a uniform pitch, p, as shown, and the depth of theoptical structures40 may decrease to naught toward thesecond edge38. This pattern, as will be discussed in more detail below, may be produced with any tool type.

It should be appreciated that using ray tracing and other analysis techniques, it is possible to determine particular arrangements for theoptical structures40 and thepattern42 that best correct particular observed non-uniformities in the output of thelightguide16. That is, one or more of the characteristics of theoptical structures40 and thepattern42 may be tailored to correct a particular non-uniformity. As described above, in connection with firstlight redirecting element26, theoptical structures40 and thepattern42 provided optical power to the output of thelightguide16 near theinput surface21 in order to mask non-uniformities that may occur near theinput surface21. Less or no optical power is provided away from theinput surface21 as fewer or less intense non-uniformities are typically observed from thelightguide16 farther from theinput surface21. In this manner, optical power is provided where most needed to mask or soften non-uniformities, while less optical power is provided where there may be fewer non-uniformities to mask. Moreover, optical power may be added virtually anywhere to the output of the lightguide by adding optical structures and/or varying the characteristics of the optical structures. Furthermore, the addition of optical power need not be uniform. Instead, optical power may be added, as necessary, to discrete regions of the lightguide output if necessary to help mask a defect or create a particular optical effect.

Planar light guides, and some wedge light guides that operate using frustrated TIR, may include an extractor pattern on a back surface of the lightguide. Typically, the extractor pattern is a pattern of white dots disposed on the back surface of the lightguide. Light incident to one of the dots is diffusely reflected by the white dot, and a portion of this reflected light is caused to exit the light guide. In spite of the diffuse nature of this method of extracting light from the lightguide, the pattern of dots may itself be visible in the lightguide output. Thus, to hide the dot pattern, additional diffusion is typically provided.

With reference toFIG. 3, anextractor film50 is shown. Formed in asurface52 of the extractor film are a plurality ofoptical structures54 disposed in apattern56. Theoptical structures54 are arranged essentially to replace the white dot pattern for providing extraction of light from the lightguide. While shown inFIG. 3 as circles or dots having varying diameters d1 and d2, theoptical structures54 are not collectively limited to any particular shape nor are they limited to any one particular shape within thepattern56. Therefore, theoptical structures54 may be prisms, lines, dots, squares, ellipses or generally any shape. Moreover, theoptical structures54 may be spaced very closely together within thepattern56, much more so than the dots within a dot pattern may be spaced and, for example, within about 50-100 μm of each other. This very close spacing of theoptical structures54 eliminates or reduces the need for diffusion in the output of the lightguide that is ordinarily necessary to hide the pattern of white dots. It is also possible to change the slope of the lightguide at a micro-level. That is, the slope of the lightguide may be locally increased or decreased at the micro-level. When a light ray hits a higher positive slope, it will be extracted from the lightguide faster than if it hit the nominal wedge angle.

While so far discussed in terms of optical films, the invention has application to the lightguide wedge itself. Referring toFIGS. 4 and 5, alightguide60 has in aninput surface62, and anoutput surface64 and aback surface66. Theinput surface62 is arranged to be disposed adjacent a light source (not depicted) to provide a source of light incident to theinput surface62. The light incident to theinput surface62 is extracted out of theoutput surface64 as a result of frustrated TIR within thelightguide60. As discussed above, it is common for there to be non-uniformities in the light output from thelightguide60, particularly near theinput surface62.

With continued reference toFIGS. 4 and 5, diffusion is added to theback surface66 of thelightguide60 and is further adjusted in intensity extending away from theinput surface62. That is, theback surface66 is formed with in-phaseoptical structures68 having amplitude A1 to provide diffusive extraction near theinput surface62. The pattern can also be non-tapering, i.e., constant amplitude A1 or A2, over the entire surface, increasing from naught, i.e., A2 greater than A1, randomly varying, or distributed in discrete regions. It is also possible for the optical structures to be out-of-phase, such asoptical structures68′ formed in aback surface66′ of thelightguide60′ shown in FIG.6. It will be appreciated that patterns of optical structures may also be formed in theoutput surface64 either separately or in conjunction with a pattern formed in theback surface66. The overall purpose of providing the optical structures is to achieve an effect that minimizes non-uniformities of the lightguide output wherever they may occur, and for thelightguide60 shown inFIGS. 4 and 5, the non-uniformities appear primarily adjacent theinput surface62.

With reference toFIG. 5, theoptical structures68 may be formed on asurface72 of anoptical film70 having a varying characteristic such as a pitch decreasing from P1 to P2. Theoptical film70 may then be coupled to the wedge structure of thelightguide60 using ultraviolet (UV) curing, pressure sensitive or any other suitable adhesive. Alternatively, the wedge may be molded in bulk to include theoptical structures68 in theback surface66.

As will be more generally appreciated from the foregoing discussion, virtually any configuration of optical structures may be formed into an optical film, and the optical film coupled, for example by bonding, to a lightguide or other bulk optical element. For example, glare reduction, anti-wetout, Fresnels, and virtually any other structure that may be formed in a surface of an optical film may be easily replicated into the film and then the film coupled to another optical element.

Films incorporating programmed optical structures may be manufactured using a microreplication process. In such a manufacturing process, a master is made, for example by cutting the pattern into a metal roll, and the master is used to produce films by extrusion, cast-and-cure, embossing and other suitable processes. Alternatively, the films may be compression or injection molded or roll formed. A preferred apparatus and method for microreplication is described in the commonly assigned U.S. patent application entitled “Optical Film With Defect-Reducing Surface and Method of Making Same,” Ser. No. 09/246,970, the disclosure of which is hereby expressly incorporated herein by reference.

As an example of the above-described feature, and with reference toFIG. 7, a linear Fresnel lens orprism80 has a substantially planar input surface82 and anoutput surface84. Theoutput surface84 is formed withlens structures86 and superimposed on thelens structures86 are additionaloptical structures88. Theoptical structures88 have characteristics, such as amplitude A1 and A2, period P1 and P2, and aspect ratio, that vary from afirst edge90 of thelens80 to asecond edge92 of thelens80. Thelens80 may be formed in bulk, or as shown inFIG. 7, thelens structures86 including theoptical structures88 may be formed on afilm94 that is then bonded to a bulkoptical substrate96.

Referring now toFIG. 8, shown graphically is afilm100 containing a varyingamplitude pattern102 that was formed using a “V” shaped tool. Thepattern102 may be formed on a top and/or bottom surface of thefilm100. Likewise, the pattern may be formed in a wedge or slab. Thefilm100 has afirst edge104 and asecond edge106. Extending from thefirst edge104 toward thesecond edge106 are a plurality ofoptical structures108 arranged in thepattern102. Eachoptical structure108 may have a number of characteristics, such as amplitude, period and aspect ratio. Thepattern102 may also have characteristics, such as for example, a pitch, p, defining a spacing betweenoptical structures108. Theoptical structures108 inFIG. 8 are shown having amplitude variation. In application of thefilm100, the grooves may be arranged such that variation in amplitude is perpendicular to a lightsource of the lightguide incorporating thefilm100.

With continued reference toFIG. 8, it is observed that within thepattern102, theoptical structures108 are formed with larger amplitude, A1, at thefirst edge104 and decrease in amplitude, A2, toward thesecond edge106. The larger amplitude produces more optical power along the groove axis because of the higher surface slopes. The optical power of this pattern then decreases as a function of the distance from thefirst edge104. This arrangement of theoptical structures108 and thepattern102 is purposeful. Theoptical structures108 may also be formed with a larger pitch, P1, at the first edge103 and decrease in pitch, P2, toward thesecond edge106.

With reference toFIGS. 9 and 10,films110 and112, are shown respectively. Eachfilm110 and112 has the same characteristics asfilm100, and like reference numerals are used to describe like elements therebetween. As opposed to the pattern created by using a “V” shaped tool, thefilm110,FIG. 9, has apattern114 ofoptical structure116 that is formed using a curved nose tool. Thefilm112,FIG. 10, has apattern118 ofoptical structures120 that is formed using a square nose tool. Thepatterns114 and118 are arranged as described to provide optical power in the surface or surfaces of thefilms110 and112. It will be appreciated that virtually any tool configuration may be used with the particular tool being selected to achieve a desired amount and form of optical power in the surface or surfaces of the film.

In thelightguide121 illustrated inFIG. 11, afirst pattern122 ofoptical structures124 is formed in abottom surface126 and asecond pattern128 ofoptical structures130 is formed in atop surface132 of thewedge134. Thefirst pattern122, having varying pitch decreasing from pitch P1 to pitch P2, may be arranged to facilitate the extraction of light from thewedge134, while thesecond pattern128 may be arranged to mask non-uniformities in the light output from thewedge134. It will be appreciated, however, that the patterns implemented in thewedge134 will depend on the desired light output to be achieved from thewedge134. Moreover, as described above, thepatterns122 and128 may be formed first in an optical film that is later coupled to thewedge134, for example, by bonding. In another form, surfaces122 and128 are injection molded with the wedge.

Still other modifications and alternative embodiments of the invention will be apparent to those skilled in the art in view of the foregoing description. This description is to be construed as illustrative only, and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. The details of the structure and method may be varied substantially without departing from the spirit of the invention, and the exclusive use of all modifications which come within the scope of the appended claims is reserved.

Claims (19)

1. An optical film for use with an edge illuminated lightguide having an input edge surface and an output surface, the optical film comprising:

a first surface;

a second surface opposing the first surface;

a plurality of prism grooves formed in the first surface, the prism grooves each having a groove axis and each groove axis being substantially parallel to each other groove axis, wherein the groove axes are arranged to be disposed substantially perpendicular to the input edge surface; and

each of the plurality of prism grooves being formed to include a plurality of optical structures, the plurality of optical structures to provide optical power to the prism groove, each optical structure having a characteristic, and the characteristic varying as a function of the location along the groove axis from the input edge surface.

2. The optical film ofclaim 1, wherein the characteristic comprises one of the group of characteristics comprising: amplitude, pitch and aspect ratio.

3. The optical film ofclaim 1, wherein the optical structures comprise optical structures selected from the group of optical structures comprising: prisms, lines, dots, squares and ellipses.

4. The optical film ofclaim 1, wherein the characteristic comprises amplitude, and wherein the amplitude varies along the groove axis.

5. The optical film ofclaim 1, wherein variation in the optical structures of adjacent prism grooves is one of in-phase and out-of-phase.

6. The optical film ofclaim 1, wherein the optical power is arranged to be greater along the groove axis at a location arranged to be nearer the input edge surface.

7. The optical film ofclaim 1, wherein the optical structures comprise discrete optical structures formed in the prism groove.

8. The optical film ofclaim 1, wherein the optical structures comprise continuous structures formed along the length of the prism groove.

9. The optical film ofclaim 1, wherein the optical power tapers from a first value to a second, lesser than the first value along the groove axis from a location on the groove axis arranged to be nearer the input edge to a location on the groove axis arranged to be farther from the input edge.

10. An optical film comprising a surface, a first edge and an opposing second edge, grooves formed in the surface extending from the first edge to the second edge, each groove having a groove axis and the groove axes being substantially aligned, optical structures formed on each of the grooves, the optical structures providing optical power to the grooves, and the optical structures having a characteristic that varies along the groove as a function of the location of the optical structure on the groove between the first edge and the second edge.

11. The optical film ofclaim 10, wherein the characteristic comprises one of the group of characteristics comprising: amplitude, pitch and aspect ratio.

12. The optical film ofclaim 10, wherein the optical structures comprise optical structures selected from the group of optical structures comprising: prisms, lines, dots, squares and ellipses.

13. The optical film ofclaim 10, wherein the characteristic comprises amplitude, and wherein the amplitude varies along the groove axis.

14. The optical film ofclaim 10, wherein variation in the optical structures of adjacent prism grooves is one of in-phase and out-of-phase.

15. The optical film ofclaim 10, wherein the optical power is arranged to be greater along the groove axis at a location arranged to be nearer the input edge surface.

16. The optical film ofclaim 10, wherein the optical structures comprise discrete optical structures formed in the prism groove.

17. The optical film ofclaim 10, wherein the optical structures comprise continuous structures formed along the length of the prism groove.

18. The optical film ofclaim 10, wherein the optical power tapers from a first value to a second, lesser than the first value along the groove axis from a location on the groove axis arranged to be nearer the input edge to a location on the groove axis arranged to be farther from the input edge.

19. A method of reducing defects in a light output of a backlight, the method comprising:

providing an optical element in the back light system, the optical element having a surface, a first edge and an opposing second edge, grooves formed in the surface extending from the first edge to the second edge, each groove having a groove axis and the groove axes being substantially aligned, optical structures formed on each of the grooves, the optical structures providing optical power to the grooves, and the optical structures having a characteristic that varies along the groove as a function of the location of the optical structure on the groove between the input edge and the second edge; and

arranging the optical element such that the grooves are aligned substantially perpendicular to a light source of the backlight and in an output path of the light source.

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